FR2676414A1 - PNEUMATIC SERVOMOTOR. - Google Patents

Abstract

The invention relates to a pneumatic brake booster comprising a casing (10) inside which there is a piston (20), formed of a rear tubular part (22) supporting a skirt (14) and which , using a rolling membrane (12), defines a front chamber (16) permanently connected to a vacuum source and a rear chamber (18) selectively connected to the front chamber (16) or to the atmosphere by valve means (20a, 32a, 40) actuated by a control rod (34) capable of resting, via a plunger (32), on one of the faces of a disc of reaction (58) integral with a push rod (56), the valve means (20a, 32a, 40) comprising a valve (40) cooperating with a first valve seat (32a) formed on the plunger (32) and with a second valve seat (20a) formed on the piston (20). According to the invention, the plunger (32) and the piston (20) comprise a second valve means (65, 70).

Description

PNEUMATIC SERVOMOTOR

  The present invention relates to pneumatic actuators, and more particularly of the type which are used to provide a

  brake assist for motor vehicles.

  Servomotors of this type conventionally comprise a piston comprising a rear tubular part and a skirt and which, using a rolling membrane, defines a front chamber permanently connected to a vacuum source and a rear chamber selectively connected to the front chamber or to the atmosphere by a valve means actuated by a control rod capable of being supported, by means of a plunger on one of the faces of a reaction disc secured to a push rod, the valve means comprising a valve cooperating by an active part with a first valve seat formed on the plunger and with a second valve seat formed on the piston. Such servomotors, if they function satisfactorily, nevertheless have some drawbacks. This is how, during braking, the passage offered to atmospheric air towards the rear chamber is reduced, which slows the movement of the air in direction

  of the rear chamber and increases the response time of the actuator.

  Different compromises have been proposed so that this response time is acceptable for so-called "normal" braking where only a slowing down of the vehicle is desired. However, this response time is always much too long during sudden braking or an intervention.

  quick enslavement is desired.

  The present invention therefore aims to provide a booster whose response time is as short as possible during sudden braking For this purpose, the invention relates to a booster in which additional air passages between the the atmosphere and the rear chamber are open in the event of sudden braking. According to the present invention, the plunger and the piston comprise a

second valve means.

  Other purposes, features and advantages will emerge from the

  description which follows of an embodiment of the present invention,

  given without limitation, with reference to the accompanying drawings in which -2- The Single Figure is a side view, in longitudinal section, representing the central part of a pneumatic brake booster produced conventionally in half lower of the Figure, and produced in accordance with the present invention

  according to the upper half of the Figure.

  The lower half of the Figure shows part of a brake booster intended to be placed in the usual way between the brake pedal of a vehicle and the master cylinder controlling the hydraulic braking circuit of this vehicle By convention , the front of the servomotor is the part of the latter facing the master cylinder and the rear of the servomotor the part

facing the brake pedal.

  The actuator comprises an outer casing 10 in the form of

  shell, having a symmetry of revolution around an axis XX '.

  Only the rear central part of this envelope 10 is shown

on the face.

  A flexible confusing elastomer membrane 12, reinforced in its central part by a metal support disc 14 also called skirt, defines inside the space delimited by the envelope 10 a front chamber 16 and a rear chamber 18 The peripheral edge outside (not shown) of the membrane 12 is tightly fixed on the outer casing 10 The inner peripheral edge of this same membrane ends in a bead received in a sealed manner in an annular groove formed on the outer peripheral surface of a

  hollow assistance piston 20 arranged along the axis XX 'of the servomotor.

  This hollow piston 20 extends towards the rear in the form of a tubular part 22 which passes in leaktight fashion through the rear wall of the casing 10 The tightness of this crossing is ensured by a reinforced annular seal 24 which is fixed by a ring 26 in a tubular central part extending the wall towards the rear

back of envelope 10.

  A compression spring 28 interposed between the piston 20 and the front wall (not shown) of the outer casing 10 normally maintains the piston 20 and the skirt 14 in a rear position of rest illustrated in the Figure, in which the rear chamber 18

  presents its minimum volume and the front chamber 16 its maximum volume.

  In its central part located between the tubular rear part 22 and the front part in which the membrane 12 and the skirt 14 are fixed, the piston 20 has a bore 30 in which is received in sliding a plunger 32 also having a symmetry of revolution around of axis XX 'The front end of a control rod 34 of the servomotor, also arranged along the axis X-X', is pivotally mounted in a blind bore 33 formed at the rear of the plunger 32. -3 - The rear end (not shown) of this rod 34, which projects outside the tubular part 22 of the piston 20, is controlled directly by the vehicle brake pedal (not shown). The annular space 36 delimited between the control rod 34 and the tubular part of the piston 20 opens to the outside atmosphere at the rear of the booster, for example by means of an air filter. same annular space can communicate with the rear chamber 18 through a radial passage 38 formed in the central part of the piston, and passages 39 formed between the plunger 32 and the bore 30 of the piston 20 when assistance means

  controlled by the plunger 32 are actuated.

  Conventionally, these assistance means comprise a three-way valve comprising an annular valve 40 mounted in the tubular part of the piston and two annular valve seats 20 a and 32 a formed respectively on the central part of the piston 20 and on the plunger. 32. The valve 40 constitutes the front end, of smaller diameter, of a flexible elastomer sleeve, the rear end of which ends in a bead sealingly mounted inside the tubular part 22 of the piston 20 Ce bead is held in place by a metal cup 42, on which rests a spring

  compression 44 tending to move the valve 40 forward.

  The annular valve seat 32a is formed on the rear end face of the plunger 32 Similarly, the annular valve seat 20a is formed on the rear end face of the central part of the piston 20, around the seat 32 a Depending on the position of the plunger 32 inside the piston 20, this arrangement allows the valve to be in constant sealing contact with at least one of the seats of

  valve 32 a and 20 a under the action of spring 44.

  A second passage 46 is formed in the central part of the piston, approximately parallel to its axis XX ′, to make the front chamber 16 of the booster communicate with an annular chamber 48 formed around the valve 40, inside the part tubular 22 of the piston 20 When the plunger 32 occupies its rear position of rest illustrated in the Figure, in which the valve 40 is in leaktight support on the seat 32 a of the plunger 32 and separated from the seat 20 a of the piston 20, the front chambers 16 and rear 18 of the booster thus communicate with each other through passage 46, the annular chamber

48 and passage 38.

  In a manner which is also conventional, at least one stop member 50 mounted in the central part of the piston 20 delimits the axial stroke of the plunger 32 inside the latter. The plunger 32 is normally held in the rear position. rest defined by the member 50 by means of a compression spring 52 interposed between the cup 42 and a washer 54 itself resting on a shoulder formed on the rod

34.

  In its central part, the piston 20 comprises an annular front face 20 b at the center of which opens the bore 30 This annular front face 20 b of the piston 20 acts on a rear face of a push rod 56, through a reaction disc 58 made of a deformable material such as an elastomer More specifically, the push rod 56 and the reaction disc 58 are arranged along the axis XX ′ of the booster, in the extension of the control rod 34 and of the plunger 32 The reaction disc 58 is capped by a cover 60 integral with the push rod 56 and centered on the axis XX 'of the booster and cooperating with an annular groove formed on the central part of the piston 20, around the annular front face 20 b of the latter. The operation of this known servomotor is conventional and can

  be briefly described as follows.

  When the servomotor is installed on a vehicle, the chamber

  before 16 permanently communicates with a vacuum source.

  Initially, the effort made on the brake pedal by the driver has the effect of equalizing the prestressing effort of the spring 52 minus the prestressing effort of the spring 44 During the next slight displacement, the control rod 34 and of the plunger 32, the valve 40, under the action of the spring 44, follows the seat 32 a of the plunger 32, until it comes into contact with the seat 20 a of the piston; the front 16 and rear 18 chambers of the servomotor are then isolated one

the other.

  In a second phase of the actuation of the brake, the plunger 32 is moved sufficiently forward so that the valve 40 is in leaktight contact with the seat 20a of the piston and begins to move away from the seat 32a of the plunger. these conditions, the rear chamber 18 of the booster is isolated from the front chamber 16 and enters into communication

with the atmosphere.

  The rear chamber 18, which previously communicated with the front chamber 16 and was therefore under reduced pressure, therefore sucks air at atmospheric pressure through the radial passage 38, the passages 39 formed between the plunger 32 and the bore 30 of the piston 20, and the valve passage of small cross section between the valve 40 and the plunger seat 32 a. There therefore follows a significant obstacle to the passage of air due to the weakness of the interstice of the valve passage. 32 a-40, and therefore a significant response time, which, if it is not very sensitive in braking where only the vehicle slowing down is desired, can become

  prohibitive during sudden braking.

  It is therefore understood that it is highly desirable to produce a servomotor o, for a stroke of the control rod 34 corresponding to sudden braking, the section offered to the passage of air is as large as possible so that the response time is the lowest

  possible without increasing the dead stroke of the actuator.

  This object is achieved thanks to the invention including an example of a mode of

  realization is shown in the upper half of the Figure.

  It can be seen in the upper half of the Figure that the piston 20 and the plunger 32 have been modified for this purpose. According to the invention, the plunger 32 slides in leaktight manner in the bore 30 thanks to two seals 62 and 64 , arranged for example in the rear part of the plunger 32 In a portion 65 of the central part of the piston 20 where the plunger is received in sealed sliding, axial passages 66 are made making the annular chamber 48 formed around the valve 40 communicate with a volume annular 67 formed between a groove 68 inside the bore 30 and a groove 69 formed at the periphery of the plunger 32 opposite the groove 68 In this volume

  annular 67 opens the radial passage 38 towards the rear chamber 18.

  Radial passages 70 are made in the plunger 32, opening on the one hand into the blind bore 33 of the plunger 32, and on the other hand at the

  outer periphery of the plunger 32 between the two seals 62 and 64.

  The operation of this servomotor is easily deduced from the foregoing explanations. The first actuation phase, during which the front chambers 16 and rear 18, are isolated one of

  the other is identical to that described above.

  In a second phase of normal actuation of the brake, the rear chamber 18 of the booster is placed in communication with the atmosphere by the radial passage 38, the annular volume 67, the axial passages 66 and the valve passage 32 a-40 II. ensues an imbalance of the pressures on the two faces of the skirt 14 of the piston, giving rise to an assistance force transmitted by the piston 20 to the push rod 56 More precisely, this assistance force is exerted by the face annular front 20 b of the piston 20 on the reaction disc 58 The latter transmits part of the assistance force to the push rod 56 and uses another part of this force to deform backwards in the space included between bore 30 and plunger 32 This deformation continues until the deformed part of the

  reaction disc 58 comes into contact with the front face of the plunger 32.

  During this phase which corresponds to what is called the jump of the servomotor, the piston 20 and the plunger 32 advance together. At the end of this phase, a reaction begins to arise on the plunger 32 and the push rod 34, causing the plunger 32 to move back relative to the piston

  , and therefore causing the closure of the valve passage 32 a-40.

  -6- It can therefore be seen that, during normal braking, the servomotor

  the invention behaves in a conventional manner.

  By cons, in a sudden brake actuation phase, a sudden force is exerted on the control rod 34 It follows a rapid advancement of the plunger 32 in the bore 30 The first phase

  isolation of the front and rear chambers takes place conventionally.

  Then the valve passage 32 a-40 opens, allowing air at atmospheric pressure to enter the rear chamber 18 via the passages 66, the annular volume 67 and the passage 38 The pressure in the rear chamber therefore increases , creating an assistance force on the piston 20 However, given the suddenness of the actuation, during the increase in pressure in the rear chamber and that of the assistance force, the plunger 32 continues to advance in the piston further than it did during normal actuation It follows that the valve passage 32 a-40 is more open, that is to say that it offers an air passage section more important than in a

  normal braking, but which is still considered too insufficient.

  According to the invention, a second passage is offered to air at atmospheric pressure to reach the rear chamber 18 In fact, the plunger 32 having penetrated into the piston 20 by an amount greater than that of normal braking , the seal 62, carried by the plunger 32 and located furthest forward, comes in front of the portion 65 of the piston 20 The sealing of the radial passage 70 relative to the portion 65 is no longer ensured, and the passage 70 is then open into the annular volume 67, the portion 65 of the piston 20 and the radial passage 70 playing the role of a slide valve That is to say that, in this sudden braking situation, the air at atmospheric pressure is admitted into the rear chamber of the booster by the valve passage 32 a-40, the passages 66, the annular volume 67 and the radial passage 38, and also by the radial passages 70 of the plunger, the volume annular 67 and the radial passage 38 We see therefore that in c In the event of extreme braking, the radial passage 38 and the annular volume 67 are connected to the atmosphere by two sources: the passages 66 and the passages 70 This obviously results in a greater influx of air towards the rear chamber, including one faster increase in assist force, and therefore a decrease in response time, which was

the goal.

  During this abrupt operating phase, the disc

  reaction 58 was also requested by the front face 20 b of the piston.

  But during its rearward deformation, it very quickly encountered the front face of the diver who had advanced enough to discover

the radial passages 70.

  -7- The reaction of the push rod 56 was therefore very quickly transmitted to the control rod 34 When the driver has reached the maximum braking force, or the desired force, what he feels when "touched" of the brake pedal, it maintains its braking force constant or releases it, which is retransmitted to the control rod The reaction disc 58 then tends to return to a position of equilibrium for the force required The front annular face 20a of the piston 20 always providing the same force since the difference in pressures on the two faces of the skirt 14 has not varied, the reaction disc 58 will tend to deform further backwards, thus pushing the plunger 32 relative to the piston 20 The passages 70 will then be closed again, and the actuator will return to normal operation. The positions relative to the rest of the plunger 32 and of the piston 20 are defined by the stop member 50. It will therefore be possible to determine the distance between the axis of the passages 70 and the front edge of the portion 65 of the piston 20 at rest, so that the passages 70 remain covered by the portion 65 during normal braking and are discovered during sudden braking. Such a distance may vary depending on the

  characteristics that one wishes to give to the servomotor thus equipped.

  It is therefore clear that a servomotor has been produced, the operation of which is conventional for normal conditions of use, and o additional air passages are opened during sudden braking, to reduce the response time of the servomotor in these

extreme conditions.

  Of course, the invention is not limited to the embodiment which has been described by way of example, but is capable of receiving numerous modifications which will appear to those skilled in the art. additional air can be formed on another part of the plunger and if the sliding of the plunger in the piston can be achieved perfectly, the use of gaskets on the plunger on either side of the passages Radials may be unnecessary. On the other hand, the invention may also be applied to servo motors in tandem or with additional assistance chamber. It will also be possible to further reduce the response time of the servomotor for sudden braking by connecting the blind bore 33 from the plunger 32 to a source of air at a pressure higher than atmospheric pressure In this way, for normal braking where only the passage 32 a-40 is open towards the rear chamber, this is supplied with air at atmospheric pressure in a conventional manner On the other hand, for sudden braking, the passages 70 are also open, the rear chamber will also be supplied with air at a pressure higher than atmospheric pressure, hence faster filling of this chamber, and a corresponding reduction in response time In the latter case, it is possible to provide a non-return valve preventing the passage of air from passages 70 to the

valve passage 32 a-40.

-8-

Claims (7)

  1 pneumatic brake booster comprising an envelope (10) inside which is a piston (20) formed by a rear tubular part (22) supporting a skirt (14) and which, using a confusing membrane (12) defines a front chamber (16) permanently connected to a vacuum source and a rear chamber (18) selectively connected to the front chamber (16) or to the atmosphere by a valve means (20a, 32a, 40) actuated by a control rod (34) capable of pressing, via a plunger (32), on one of the faces of a reaction disc (58 ) integral with a push rod (56), the valve means (20a, 32a, 40) comprising a valve (40) cooperating with a first valve seat (32a) formed on the plunger (32) and with a second valve seat (20a) formed on the piston (20), characterized in that the plunger (32) and the piston   (20) comprise a second valve means (65, 70).   2 Servomotor according to claim 1, characterized in that the plunger (32) slides in leaktight manner in a portion (65) of the piston (20).   3 Booster according to claim 2, characterized in that axial passages (66) are formed in the portion (65) of the piston (20).   4 Booster according to claim 3, characterized in that the axial passages (66) of the portion (65) of the piston connect the rear chamber (18) of the booster selectively to the front chamber (16) or to the atmosphere. Booster according to claim 2, characterized in that the second valve means (65, 70) is formed between the portion (65) of the piston   (20) and radial passages (70) formed in the plunger (32).   6 Booster according to claim 5, characterized in that the radial passages (70) formed in the plunger (32) are covered by the portion (65) of the piston (20) when the booster is at rest   and during a normal braking action.   7 Booster according to claim 5, characterized in that the radial passages (70) formed in the plunger (32) are uncovered by the portion (65) of the piston (20) during an abrupt braking action. - 9- 8 Actuator according to claim 7, characterized in that the radial passages (70) connect the rear chamber (18) of the actuator   to the atmosphere during a sudden braking action.   9 Booster according to any one of claims   above, characterized in that in the servomotor rest position, the axis of the radial passages (70) is at a distance   predetermined front edge of the portion (65) of the piston (20).   Booster according to any one of the claims   previous, characterized in that seals (62, 64) are arranged at the outer periphery of the plunger (32) in front and in   rear of the radial passages (70).